1. Field of the Invention
The present invention relates to the field of non-volatile integrated circuit analog signal recording and playback storage devices. More particularly, the present invention relates to detecting an end of message recorded onto an array of memory cells.
2. Description of Related Art
When an input signal is recorded onto an array of memory cells, it is necessary to determine the end of the recording during playback. FIG. 1 illustrates a typical memory array 100 for implementing a prior art end-of-message ("EOM") marker. Referring to FIG. 1, the memory array 100 has N rows and M columns and includes a plurality of dedicated EOM columns 110, typically located at both ends of the memory array 100, for storing and indicating an EOM condition. Each EOM column corresponds to a plurality of columns of memory cells (referred to as a scan) By way of example, for a 160.times.800 memory array, four (4) EOM columns are used, one for every two hundred columns of memory cells (e.g., two EOM columns at each end of the array). Thus, during recording of an input signal, the record operation continues to record the input signal until the end of a scan is reached. At that time, if a stop record command is detected, the recorder stops and the EOM cell corresponding to the scan just recorded is programmed to indicate an EOM condition, even though the stop record command may have been issued in the middle of the scan. However, if at the end of the scan, a record command is not detected, the record operation continues to record the input signal, without checking for a stop record command, until the end of the next scan, and so on.
During playback, the playback operation plays the input signal recorded onto the array of memory cells. At the end of a scan, the playback operation checks the EOM cell corresponding to the scan just played back and if it indicated an EOM condition, the playback operation stops. If an EOM condition is not present, the playback operation continues to play back the next scan. At the end of the next scan, the EOM cell corresponding the scan just played back is checked. If an EOM condition is present, the playback operation stops. The playback operation continues until an EOM condition is detected or the end of the memory array is reached.
This method of recording and playback is not desirable if a shorter message or more than one message is preferred. Moreover, since dedicated cells are reserved solely for the EOM marker, any imperfections in the cells may cause an error in detecting the EOM, given the fact that the chance of having a bad cell may be as low as one in one or two hundred. Therefore, it is desirable to provide a redundant method of detecting an end of message due to the fact that the array of memory cells may contain imperfections. It is further desirable to end the recording immediately after a stop command is detected. If the recording operation is allowed to continue for some time after the stop command is detected, then unwanted sounds are likely to be recorded such as the sound of the stop record switch.
Accordingly, there is a need in the art for an apparatus and method of indicating and detecting an EOM in analog storage devices or any type of multi-level storage devices.